Current Fluctuations in Nanopores Reveal the Polymer-Wall Adsorption Potential

Modification of surface properties by polymer adsorption is a widely used technique to tune interactions in molecular experiments such as nanopore sensing. Here, we investigate how the ionic current noise through solid-state nanopores reflects the adsorption of short, neutral polymers to the pore surface. The power spectral density of the noise shows a characteristic change upon adsorption of polymer, the magnitude of which is strongly dependent on both polymer length and salt concentration. In particular, for short polymers at low salt concentrations no change is observed, despite the verification of comparable adsorption in these systems using quartz crystal microbalance measurements. We propose that the characteristic noise is generated by the movement of polymers on and off the surface and perform simulations to assess the feasibility of this model. Excellent agreement with experimental data is obtained using physically motivated simulation parameters, providing deep insight into the shape of the adsorption potential and underlying processes. This paves the way toward using noise spectral analysis for in situ characterization of functionalized nanopores.

Figure 1

Measuring changes in ionic current noise upon introducing PEG into a glass nanopore. (a) Experimental schematic illustrating the insertion of polymer molecules (PEG) into the pore using EOF. (b) Short sections of the ionic current traces before and after the introduction of PEG into solution. The traces are digitally filtered to a 5 kHz bandwidth and centered to their own mean current to emphasize differences in noise properties. (c) Concomitant changes in the power spectral density of the noise.

Figure 2

Verifying PEG adsorption with QCM D. Frequency ($\mathrm{\Delta }F$) and dissipation ($\mathrm{\Delta }D$) shifts from QCM D measurements for $50\mu \mathrm{M}$ PEG 8000 in 50 mM (right) and 500 mM (left) KCl solutions. Three harmonics are shown, all rescaled by their harmonic number $n$.

Figure 3

Top: The effect of PEG length on excess noise. Spectra are shown for PEGs 1000, 8000, and 20 000 in 150 mM KCl. Bottom: The excess noise, $S-S_0$, for all systems. Curves are shown only for frequencies where $S_0<S$. The range of corner frequencies obtained by fitting to Eq. (1) are shown in gray. A typical limiting slope of 1.4 is marked.

Figure 4

(a) Variation of the corner frequency $f_0$ and limiting exponent $\gamma$, with adsorption potential parameters $\alpha$ and $r^{*}$ (inset), obtained from simulated spectra. Experimental ranges of $f_0$ for each PEG weight are shown in gray (top), and the observed limit of $\gamma \sim 1.4$ is marked (bottom). (b) Comparison of experimental (red and black) and simulated (blue) spectra, for all parameter combinations. Confidence intervals for simulated spectra are shaded. Columns have constant PEG weight and rows have constant salt concentration, as indicated.